Flame-arresting vent valve

ABSTRACT

This invention relates to a flame-arresting vent valve which is positioned to communicate with the uppermost portion of the motor casing of a submersible pump. The vent includes a pressureactuated valve member and a flame arrester made of porous material to prevent the propagation of flame across the valve. This invention is especially suited for use in pumps which deliver combustible fluids.

United States Patent Bryant et a1.

[451 Jan. 18, 1972 FLAME-ARRESTING VENT VALVE Inventors: Gerald T. Bryant, Corona; James L.

Mayiield, Garden Grove, both of Calif.

Air Reduction Company, Incorporated, New York, NY.

Filed: Apr. 4, 1969 Appl. No.: 813,574

Assignee:

US. Cl ..4l7/53, 417/424, 310/87, 310/88 Int. Cl. ..F04b 17/00, F04b 35/04, H02k 5/10 Field ofSearch .....103/89, 87 D, 87 E, 111 D, 103/221; 222/189; 220/88; 310/88, 87; 417/366,

References Cited UNITED STATES PATENTS La Font Jennings ..103/87 2,839,006 6/1958 Mayo 103/87 2,972,308 2/1961 Haggerty ...103/87 3,356,255 12/1967 Zavertnik et al.. ...220/88 2,510,632 6/1950 Hemphill ..41 7/369 FORElGN PATENTS 0R APPLICATIONS 664,390 6/1963 Canada ..4 1 7/366 Primary Examiner-Robert M. Walker Attorney-Francis B. Henry, Edmund W. Bopp and H. Hume Mathews [5 7] ABSTRACT This invention relates to a flame-arresting vent valve which is positioned to communicate with the uppermost portion of the motor casing of a submersible pump. The vent includes a pressure-actuated valve member and a flame arrester made of porous material to prevent the propagation of flame across the valve. This invention is especially suited for use in pumps which deliver combustible fluids.

16 Claims, 3 Drawing Figures PATENTED mu 8 m2 SHEET 1 OF 2 PATENTED JAN 1 8 1972 SHEET 2 BF 2 FIG. 2

a. r. BRYANT Z: J.L.MAVF/ELD ATTORNEV FLAME-ARRESTING VENT VALVE Heretofore, the submersible pumps which were designed to pump liquefied natural gas (LNG) or other liquefied gases such as methane, propane, butane, ethylene, ammonia, and the like, had no provision for venting the air which is normally trapped in the motor casing of the pump. In conventional storage installations in which the submersible pump is mounted in the lower portion of the storage vessel, air is trapped in at least the upper portion of the motor casing due to the fact that there is no venting system by which the air can escape from the motor casing. As the storage vessel is filled with the liquid being stored the liquid permeates the bearings of the pump and seeks its way into the motor casing wherein it compresses the air into the upper portion of the motor casing. The air remains in the upper portion of the motor casing under pressure when the pump starts and during the normal operation of the pump. This presents a safety hazard in that the air contains oxygen, which mixes with vapors from the combustible fluid which is being pumped and this gaseous mixture surrounds the rotor and stator and a portion of the electrical leads in the motor casing. In the event that a spark occurs during the startup or the operation of the pump motor there is a good possibility that the spark will result in an explosion in the interior of the motor casing due to the presence of the disassociated oxygen in mixture with the vapor of the product liquid. Explosions of this type have occurred. Fortunately, serious injury has been avoided due to the fact that the explosions have not permeated the motor casing housing but have merely destroyed the interior of the pump. If the motor casing fails, however, the flames, etc., may be transmitted into the liquid product in the interior of the storage tank possibly causing the whole storage tank to explode.

In the prior art form of submersible pumps there was no provision for effectively venting the motor casing and preventing the escape of flames from the interior of the motor casing to the exterior of the motor casing. The present invention prevents the flames from propagating into either the storage tank or the discharge lines leading to the exterior of the storage tank. I

It is the primary object of this invention to vent any and all air which is present in the motor casing. The venting takes place upon the lowering of the submersible pump into the liquid which is to be pumped. The venting also takes place when the pump is in a fixed position in the bottom portion of a storage tank and the tank is then filled with the liquid which will eventually be pumped out.

It is another object of this invention to prevent flames from leaving the motor casing of the submersible pump.

Another object of this invention is to prevent the flow of liquid through the motor casing of a submersible pump while the pump is in operation. The object is to maintain an atmosphere entirely of the vapor of the liquid being pumped in the motor casing. This prevents the continuous vaporization of the liquid in the motor casing.

It is a further object of this invention to keep the motor casing of a submersible pump substantially free of liquid, in order that the efficiency of the pump may be optimized.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawing, wherein a preferred embodiment of the present invention is clearly shown.

In the drawing:

FIG. I is a diagrammatic vertical sectional view, partly broken away, of a storage tank with a submersible pump constructed in accordance with the principles of this invention located in the bottom of the storage tank.

FIG. 2 is a side elevational view, partly broken away, partly in section, showing portions of the submersible pump and showing the location of the flame-arresting vent valve.

FIG. 3 is a side sectional view of a flame arresting vent valve showing the details thereof.

In FIG. 1 there is illustrated a cross-sectional view of the storage tank of an ocean going cryogenic liquid transport vessel. In order to simplify the disclosure of this invention the drawing is presented in a schematic form. The reference numeral 10 generally indicates the hold of the vessel which is defined by sidewalls ll, deck 12 and bottom portion 13. The pump and motor assembly 20 is positioned on the bottom of the tank and is held in position by means of struts 21, 22. As can be expected, the bottom portion of the tank collects a considerable amount of residual matter and foreign matter which would quickly contaminate the pumping system causing the malfunction of the pump, etc. In order to prevent the entrance of this material into the pumping system a screen 23 is normally positioned around the pump entrance opening. The pump normally pulls a suction through the screen 23 and discharges the liquid into a discharge conduit 24 which directs the liquid top side to a further conduit system 25 which leads the liquid off the ship to storage tanks on shore. While the invention is especially suited for use in storage tanks on board ships, the invention may also be used in storage tanks or vessels on shore, under water or under ground. For example, the pumping system with the flame-arresting vent valve may also be used in the well-known frozen hole" storage facility. The flame-arresting vent valve can also of course be used with any pumping system having the requirement that the motor casing be purged of air.

The electrical leads leading to the pumping system extend through a fluid tight conduit 26 from the deck area down to the pump and motor assembly. A suitable inlet conduit 27 is provided for filling the tank. From FIG. 1 it can be observed that as the fluid enters through conduit 27 it fills the bottom portion of the tank 10 at first and then the level of liquid rises above the pump and motor assembly 20 and fills the remaining portion of the tank 10. In normal operation the tank 10 is filled to the extent possible, this being done to reduce the free surface effect of the liquid in the tank and also to leave as little vapor space as possible in the upper portion of the tank 10. Both conduits 24 and 27 are equipped with suitable valves 28 and 29 which are used to seal off the tank 10 from the atmosphere.

In view of the fact that the details of the conduit system 25 through 27 and the tank 10 form no part of the present invention, this structure is shown in schematic form in FIG. 1. Perforated members 14, 15 are used to strengthen the interior of the tank. The perforations allow free communication between the different portions of tank 10. FIG. 1 is of course not to scale but is illustrative of the form the tanks take.

FIG. 2 illustrates in more detail the elements of the pump and motor assembly and of the flame-arresting vent valve. The pump and motor assembly is generally indicated by the reference numeral 30and comprises a pump assembly 31 and a motor assembly 32. This assembly is shown in a sectional view which is partly broken away for purposes of clarity.

The pump assembly comprises a bottom housing 35 which is bolted to the motor casing 36 by means of a plurality of bolts 37. The lowermost portion of the housing 35 contains a flared annular member 38 which supports a plurality of radially extending guide vanes 39. The guide vanes prevent the swirling of the liquid that is being sucked into the pump and serve to guide the flow of the liquid in a radial direction into the interior of the pump. A substantially cylindrical portion 40 interconnects the annular member 38 and the upper housing 35. A plurality of axially extending struts 41 interconnect the annular member 38 and the upper housing 35 for the purposes of strengthening the entire pump casing.

For purposes of simplifying the illustration the conventional screening arrangement which is positioned around the antivortex guide vane and the inlet opening to the pump is not shown in FIG. 2.

The rotating portions of the pump assembly include the inducer 45 and the impeller 46 both of which are mounted on the shaft 47. Both the inducer and the impeller may. be removed from the shaft by removing bolt 48. The inducer 45 contains helical vanes which force the fluid being pumped up into the impeller 46. The impeller forces the fluid into the diffuser 50. The liquid being pumped then passes up through annular chamber 51 and by means of passages not shown enters into discharge chamber 52. A flange 53 is positioned on the upper edge of the discharge chamber 52 and is adapted to connect with a flange on the lower end of the discharge conduit 24.

A portion of the liquid which is being discharged is taken from the conduit system 25 and passed through a mechanical filtering mechanism 55 and is then passed down through conduit 57 to opening 58 located in the upper portion of the motor casing. The opening 58 communicates with internal conduit 59 which directs the liquid to the upper bearing assembly 62. In the pump and motor assembly illustrated in FIG. 2 the bearings are lubricated by the fluid being pumped. This technique avoids the use of sealed bearings which are prone to failure in this type of environment. The liquid which is used to lubricate the bearings is filtered on deck in a removable or replaceable filter 55 so that the contaminants which pass through the screen and enter into the pumping system do no contaminate or interfere with the normal operation of the bearings.

The upper bearing assembly 62 comprises an annular stainless steel support ring 63, an angular contact ball hearing assembly 64 which is loaded by a Bellville spring 65. The outer race of the bearing contacts the ring 63 and the inner race of the bearing contacts the shaft 47.

A plurality of ports 66 are drilled through the wall of the shaft 47. For a portion of its length the shaft 47 is drilled out to fonn a chamber 67 which communicates with the conduits or ports 66. The upper portion of the chamber 67 is sealed by means of plug 68. The lower portion of the chamber 67 communicates with ports 69. As can be seen from FIG. 2 the liquid which enters through port 58 passes through inner conduit 59 and lubricates the upper bearing assembly 62. The fluid then passes through ports 66 into the shaft 47 and flows downwardly through chamber 67 and then outwardly through ports 69 into the lower bearing assembly 70. The lower hearing assembly 70 is constructed in a manner comparable to the upper bearing assembly and includes a support ring 71, a ball bearing assembly 72 and a Bellville spring assembly 73. The liquid which enters the lower bearing assembly 70 from ports 69 passes through the bearing assembly and then passes into the area of the impeller 46. Conventional wear rings 75 and 76 define a tight restriction between the rotating portion of the impeller and the adjacent housing in order that the high-pressure fluid which is generated by the impeller does not flow through the areas of the wear rings 75 and 76. Thus the fluid which passes through the bearing assembly 70 enters chamber 77 and then passes through ports 78 in the lower portion of the impeller wheel. The bearing liquid is then discharged by the impeller into the diffuser section 50 and then upward again into chamber 51 for discharge. Thus a portion of the discharge fluid is recirculated for the purposes of lubricating the bearings.

The details of the motor assembly will now be described. The motor assembly comprises a stator 80 and a rotor 81. The rotor is keyed 82 to the shaft 47. Suitable electrical leads extend from top side through fluid-type conduit 26 and pass into the conduit box 85. Suitable electrical connectors 86 are utilized to connect the incoming electrical leads to the electrical leads leading to the stator. A spacing block 87 is utilized to separate the electrical leads and to hold them in a fixed position thereby preventing the wear or fraying of the insulation which surrounds the electrical leads and the possible shorting of the electrical system. In the conventional conduit box a replaceable cover 88 is utilized so that inspection of the electrical leads and the connection 86 may be made and so that the electrical leads may be disconnected from the power source when desired. It has been found that certain electrical codes demand that the conduit box be filled with epoxy compound. This is apparently done in order to rigidly position the electrical connection 86 and the electrical leads leading into the connection in order to prevent short circuits, etc. The epoxy is shown in FIG. 2. The use of epoxy of course presents certain problems in that it must be chipped away or cut away if the electrical leads are to be replaced or repaired or connected in another manner.

Attention is directed to the fact that the motor casing contains a cavity of considerable volume. The cavity surrounds the rotor and stator and extends upward to the conduit box. If the conduit box were not packed with epoxy it would also constitute a portion of this motor casing cavity. When the motor casing is not submersed in the storage tank or other operative position the cavity normally contains air. It is the purpose of the present invention to vent this air when the pump is submersed. In order to vent the air from the motor casing cavity a flame-arresting vent valve generally indicated by the reference number 90 is positioned to communicate with the uppermost portion of the cavity. In FIG. 2 the flame-arresting vent valve is mounted on the uppermost portion of cylinder 92 which connects the conduit box 85 to the upper portion 93 of the motor casing.

If for example the pump and motor assembly of FIG. 2 is positioned as shown in FIG. 1 and the storage tank 10 is flooded with liquid, the liquid level will rise in the pump itself in that the pump directly communicates with the fluid in the storage tank. The fluid will pass through the inducer and the impeller 46 and will pass up into chamber 51 and into discharge chamber 52 and into the discharge conduit 24. The liquid will also pass through port 78 and through the lower bearing assembly, port 69, chamber 67, port 66, upper bearing assembly 62, and into the conduit 59. The liquid will rise in conduits 24 and 57 displacing the air therein which is normally vented by suitable valves topside. In normal circumstances the level of liquid in the conduits 57, 24, and in the tank 11, will be above the motor and the pump assembly. The liquid which has passed into the interior of the motor assembly will pass through flame paths 95 and 96. The flame path 95 is defined by a cylindrical surface 97 on an extension 104 of shaft 47 and by a cylindrical surface 98 machined into the upper portion of the motor casing housing. In its preferred form the flame path has a maximum clearance of 0.025 inch and is I52 inches in length. The purpose of the flame path is to prevent the propagation of a flame from the interior of the cavity to the exterior of the cavity through the bearing assembly. The flame paths effectively prevent this in the well-known manner. Flame path 96 is similarly defined by a cylindrical surface 102 machined into the lower portion of the motor casing assembly and by cylindrical surface 103 on the shaft 47. The dimensions of the flame path 96 should be essentially identical with the dimensions of flame path 95.

It can be seen that as the liquid enters the bearing assemblies it will permeate the flame paths and enter the motor casing cavity. The liquid will continue to enter the motor casing cavity due to the head of the liquid in the storage tank and the liquid will compress the air or vapor in the cavity into the uppermost portion of the cavity. In the motor casing configuration of FIG. 2 the uppermost portion of the cavity is indicated by the reference numerals and 111. If the flame-arresting vent valve 90 were not positioned as indicated in FIG. 2, the compressed air would remain in the cavity portions 110 and 111, there being no means of escape. If in fact there were no vent valve this Compressed air would remain in the cavity 110 and 111 until the pump motor was started.'Upon starting, the rotation of the rotor and the heat generated by the actuation of the pump would at least partially vaporize the liquid product in the motor cavity. The vapor formed by this vaporization would mix with the air in the cavity 110 and 111 and would result in a gaseous mixture highly susceptible to ignition if the mixture came into contact with an electrical spark, excessive heat, hot wire, or any other mechanism which would bring the mixture to its ignition temperature. Since the air that is trapped in the motors casing contains oxygen and in the embodiment shown in FIG. 2 the flame-arresting vent valve is shown mounted in an annular collar 112 which is welded or otherwise affixed to the cylinder 92. The internal portion of the collar 112 communicates with the port 113 which is drilled into the cylinder 92. As can be seen from FiG. 2 the port 113 communicates with the uppermost portion of cavity 111. Thus any air in the motor casing cavity will be displaced by liquid rising in the cavity and will vent through the port 113 through the vent valve 90. in the interests of clarity, the details of the vent valve are not shown in FIG. 2 but are illustrated in FIG. 3 which is a detailed view in cross section of the flame-arresting vent valve.

in the preferred form of the valve the body portion 120 is made from an aluminum alloy pipe plug having a hexagonal head. A portion of the periphery of the body is threaded 121 so that it can be screwed into position in the collar 112. The flame-arresting vent valve could of course be screwed into the threaded opening in the conduit box if the epoxy compound were not utilized to pack the box, or it could be threaded directly into cylindrical member 92. in the embodiment shown the wall thickness of cylinder 92 prevented the vent valve from being directly threaded through it and therefore the annular collar 112 was utilized as a support member. A hole 122 is partially drilled through the body portion 120. The body portion is then further drilled to form holes 123 and 124 of equal diameter, Note that the diameter of hole 122 is greater than the diameter of hole 123 and hole 124. The purpose for this will be explained below. The portion of the body remaining between the holes 123 and 124 is drilled centrally to form central opening 125 and is drilled toform four equally spaced openings surrounding the central opening. One of these lastmentioned ports is indicated by reference number 126. Slidably mounted through the central opening 125 is an elongated spacer 128. A bolt 129 extends through the spacer and is threaded at one end to receive lock nut 130. A flat washer 131 is positioned between the lock nut and the body portion 132. A valve assembly 140 is slidably mounted on the bolt 129. The valve assembly which is preferably made of Teflon is partially supported by washers 141 and 142. Valve 140 is biased to an open position by means of spring 145 which rests on body portion 132 and forces the valve 140 away from the valve seat 146. Teflon is utilized as the valve material due to the fact that it retains it characteristics at cryogenic temperatures and does not become brittle and hard at cryogenic temperatures (below about l F.). The type of material used depends on the temperature and characteristics of the fluid being pumped.

An annular groove 150 is machined in the surface of body 120 concentric with the hole .122. The internal rim of the groove 150 is normally axially extending but is crimped inwardly in order to finnly hold in place a block of sintered material which is positioned in the hole 122. in its preferred form the sintered material is formed from sintered bronze having a 40-micron nominal porosity. it has been found that a sintered bronze block having the aforementioned porosity and being one-half inch long is entirely suitable for preventing flame propagation when liquefied natural gas, methane, etc., is being handled. The cylindrical block of sintered bronze is positioned in the hole 122 and bottoms on the annular wall portion 152. The annular internal rim 151 is then crimped over the exposed frustoconical edge of the sintered bronze block with a crimping tool.

it is evident from FIG. 2 that the flame-arresting vent valve is positioned in such an attitude so as to allow the sintered material to directly contact the liquid which is in the Storage tank. The valve mechanism is positioned so that it is communicating directly with the contents of the motor casing cavity. With the valve 140 in an open position, liquid and vapor may pass through the flame-arresting vent valve. For example, when the storage tank is initially filled with liquid and the liquid level in the motor casing cavity rises to displace the air in the cavity, the air is forced out of the cavity through the vent valve 90. The valve assembly 140 forms no restriction for the air in that it is in its normally spring-loaded open position. The sintered material while forming a partial restriction for the passage of air has been designed with a porosity which allows the air to escape from the cavity. Thus the liquid which floods the motor casing entirely displaces the air in the motor casing. When the liquid level in the tank and the liquid level in the motor casing reaches the vent valve, the motor casing will be entirely flooded with the liquid product and no air will remain inside the motor casing.

When the pump is started up, the liquid product which is in the motor casing will begin to vaporize. This will cause an increase in pressure in the motor casing which will close the valve 140, there being a differential pressure across said valve caused by the vaporization of the liquid product and the consequent buildup of pressure in the motor casing. The vapor pressure in the motor casing will increase until it exceeds the pressure of the liquidbeing pumped through the bearinglubricating system and then the vapor will flow through the flame paths and 96 into the bearing-lubricating system and the vapor will then exit through port 78 and go into the discharge system. The excess pressure in the motor casing will be bled off until an equilibrium situation exists wherein the pressure within the motor casing is substantially equal to the pressure of the fluid being pumped through the bearinglubricating system.

Thus in the applicants system there is substantially no continuous vaporization of the liquid product in the motor casing cavity. The liquid which is initially in the motor casing cavity is of course vaporized when the pump is energized but due to the pressure which is generated and maintained in the motor casing cavity, substantially no additional liquid is allowed to bleed into the cavity through the flame paths. Consequently, there is little or no continuous vaporization of the liquid being pumped in the motor casing.

When the pump is shut down, the pressure in the bearinglubricating system drops. The higher vapor pressure in the motor casing then bleeds out through the flame paths. The motor casing vapor pressure continues to drop until valve 140 pops open allowing communication between the interior of the motor casing and the tank through the flame-arresting vent valve. The vapor in the motor casing will then be displaced by the liquid product and escape through the valve. Upon pump startup the above-described cycle repeats itself.

in its preferred form the body of the flame-arresting vent valve is formed of aluminum and the sintered material is formed of a porous bronze or other sintered material having a rating of from about 10 to about 500 microns-nominal (preferably about 40 microns or less) and a length of at least about 0.2 inch and preferably approximately one-half inch. it is understood that other suitable sintered materials could be used in place of sintered bronze; for example, sintered stainless steel or sintered copper could also be utilized. However, the micron rating of the sintered material and its length should be such that it prevents the propagation of flames through the sintered material so that any flames which do occur either in the motor casing cavity or exterior of the motor casing will not be transmitted through the sintered material. The micron rating refers to the nominal size of a particle which will pass through the material.

it is evident that if there is a flame within the motor casing cavity it will be unable to successfully traverse either the flame paths 95 and 96 or the sintered material 155.

Although sintered material having the above characteristics is preferred for the purpose of flame arresting in this invention, it is also possible to use other mechanisms for this purpose. For example one or more elongated small drilled holes in the body could be used in place of the sintered block 155. The holes would have to be of such a diameter and length so as to prevent flame propagation across the valve. The sintered material is preferred in that due to its porous nature it is less apt to plug up with contaminants, foreign matter etc.

Other porous materials (such as for example sand, etc., having flame-arresting characteristics could also be used in place of the sintered material. It is preferred, however, that the material not be loose in that this might lead to excessive wear or destruction of the bearings.

The flame-arresting vent valve can be utilized with any pumping system in which the motor cavity is flooded with the liquid being pumped. lt must always be positioned so that it communicates directly with the air which is trapped in the upper portion of the motor cavity. lt must be positioned in such a location that all of the air in the cavity has a tendency to vent through the vent valve.

The flame-arresting vent valve can also be used to vent undesired fluids from the interior of dynamoelectric machines and to arrest flame propagation either into or out of said machines. Further use of the invention may be made where it is necessary to vent apparatus and at the same time insulate the surroundings from flames generated within the apparatus.

To those skilled in the art to which our invention relates, many changes in construction and widely differing embodiments and applications of the invention may suggest themselves without departing from the spirit and scope of the invention. Our disclosure and the description herein are purely illustrative and are not intended to be in any'sense limiting.

We claim:

1. A submersible pump system comprising a pump and motor assembly positioned in a liquid storage tank, discharge means leading from said pump and motor assembly, vent means to vent the air in the motor casing of said motor assembly as the pump and motor assembly is submerged in the liquid, said vent means comprising a flame-arresting vent valve having a body portion, an opening extending though said body portion and communicating with the uppermost portions of the interior of the motor casing whereby as the liquid level rises in said motor casing the displaced air is forced through said vent means and flame-arresting means positioned in said opening to prevent the passage of flame through said opening.

2. The pump system of claim 1 having means to lubricate the bearings of said pump and motor assembly with the liquid product being pumped.

3. The pump system of claim 2 having flame path means allowing restricted communication between the interior of the motor casing of the motor assembly and the storage tank.

4. The pump system of claim 1 in which said flame-arresting means comprises a sintered metal block which extends across a portion of said opening.

5. The pump system of claim 4 in which the sintered metal block contains openings of a nominal diameter of from about to about 500 microns.

6. The pump system of claim 4 in which the sintered material is made of material taken from the class consisting of sintered bronze, sintered stainless steel and sintered copper.

7. The pump system of claim 1 in which said vent means further comprises a spring-loaded vent valve positioned in said opening between said flame-arresting means and said motor casing, said valve being normally urged toward an open position by said spring.

8. A method of starting and operating a submersible pump having a motor comprising the steps of submersing the pump in liquid, displacing the air in the motor casing of said pump with said liquid, venting the air so that it escapes from said pump, starting the pump motor, vaporizing at least part of the liquid in said casing and maintaining a pressure therein which substantially prevents the entry of further liquid into said motor casing.

9. A method as defined in claim 8 characterized by supplying liquid to bearings of the pump to lubricate the same.

10. A motor driven pump comprising a motor casing, vent means communicating with the interior of said motor casing to allow the escape of fluid from said motor casing, said vent means including flame-arresting means to prevent the passage of flame from the interior of the motor casing, said vent means further including pressure-responsive means adapted to maintain said vent means open while the pressure in said motor casing remains substantially equal to the pressure without said motor casing and to close said vent means in response to a predetermined pressure buildup in said motor casing.

11. The motor driven pump of claim 10 in which said pressure-responsive means includes a spring-loaded valve which closes in response to a buildup of pressure in the motor casing.

12. A motor-driven submersible pump comprising a pump assembly, a motor assembly having a motor casing, means to lubricate the bearings of said pump with the fluid being pumped, vent means communicating with said motor casing to pennit fluid to displace the undesired gases in said casing when the pump is submersed in fluid, said vent means including pressure-responsive means adapted to close said vent means when a predetermined differential pressure across said pressure-responsive means is reached.

13. The pump of claim 12 in which said vent means further comprises flame arresting means to prevent the passage of flame from the interior of the motor casing.

14. A motor-driven submersible pump for delivering combustible fluids comprising a pump assembly, a motor assembly having a casing, means to lubricate the bearings of the pump with the combustible fluid being pumped, vent means connected to said motor casing to vent air from said casing as said pump is submersed in said combustible fluid, said vent means including valve means responsive to pressure buildup in said casing to close said vent means.

15. A motor driven pump as defined in claim 14 in which said vent means further comprises flame-arresting means to prevent the passage of flame from the interior of the motor casing.

16. A motor-driven pump comprising a motor assembly and a pump assembly, said motor assembly including a motor casing defining a cavity, a stator and a rotor positioned in said cavity, means to lubricate the pump bearings with the fluid being pumped, vent means connected to said motor casing to vent gas in said cavity, said vent means including flame-arresting means to prevent the passage of flame through the vent means and valve means operable in response to a predetermined pressure increase in said casing to close said vent means and thereby prevent the passage of fluid through said vent means. 

1. A submersible pump system comprising a pump and motor assembly positioned in a liquid storage tank, discharge means leading from said pump and motor assembly, vent means to vent the air in the motor casing of said motor assembly as the pump and motor assembly is submerged in the liquid, said vent means comprising a flame-arresting vent valve having a body portion, an opening extending though said body portion and communicating with the uppermost portions of the interior of the motor casing whereby as the liquid level rises in said motor casing the displaced air is forced through said vent means and flamearresting means positioned in said opening to prevent the passage of flame through said opening.
 2. The pump system of claim 1 having means to lubricate the bearings of said pump and motor assembly with the liquid product being pumped.
 3. The pump system of claim 2 having flame path means allowing restricted communication between the interior of the motor casing of the motor assembly and the storage tank.
 4. The pump system of claim 1 in which said flame-arresting means comprises a sintered metal block which extends across a portion of said opening.
 5. The pump system of claim 4 in which the sintered metal block contains openings of a nominal diameter of from about 10 to about 500 microns.
 6. The pump system of claim 4 in which the sintered material is made of material taken from the class consisting of sintered bronze, sintered stainless steel and sintered copper.
 7. The pump system of claim 1 in which said vent means further comprises a spring-loaded vent valve positioned in said opening between said flame-arresting means and said motor casing, said valve being normally urged toward an open position by said spring.
 8. A method of starting and operating a submersible pump having a motor comprising the steps of submersing the pump in liquid, displacing the air in the motor casing of said pump with said liquid, venting the air so that it escapes from said pump, starting the pump motor, vaporizing at least part of the liquid in said casing and maintaining a pressure therein which substantially prevents the entry of further liquid into said motor casing.
 9. A method as defined in claim 8 characterized by supplying liquid to bearings of the pump to lubricate the same.
 10. A motor driven pump comprising a motor casing, vent means communicating with the interior of said motor casing to allow the escape of fluid from said motor casing, said vent means including flame-arresting means to prevent the passage of flame from the interior of the motor casing, said vent means further including pressure-responsive means adapted to maintain said vent means open while the pressure in said motor casing remains substantially equal to the pressure without said motor casing and to close said vent means in response to a predetermined pressure buildup in said motor casing.
 11. The motor driven pump of claim 10 in which said pressure-responsive means includes a spring-loaded valve which closes in response to a buildup of pressure in the motor casing.
 12. A motor-driven submersible pump comprising a pump assembly, a motor assembly having a motor casing, means to lubricate the bearings of said pump with the fluid being pumped, vent means communicating with said motor casing to permit fluid to displace the undesired gases in said casing when the pump is submersed in fluid, said vent means including pressure-responsive means adapted to close said vent means when a predetermined differential pressure across said pressure-responsive means is reached.
 13. The pump of claim 12 in which said vent means further comprises flame arresting means to prevent the passage of flame from the interior of the motor casing.
 14. A motor-driven submersible pump for delivering combustible fluids comprising a pump assembly, a motor assembly having a casing, means to lubricate the bearings of the pump with the combustible fluid being pumped, vent means connected to said motor casing to vent air from said casing as said pump is submersed in said combustible fluid, said vent means including valve means responsive to pressure buildup in said casing to close said vent means.
 15. A motor driven pump as defined in claim 14 in which said vent means further comprises flame-arresting means to prevent the passage of flame from the interior of the motor casing.
 16. A motor-driven pump comprising a motor assembly and a pump assembly, said motor assembly including a motor casing defining a cavity, a stator and a rotor positioned in said cavity, means to lubricate the pump bearings with the fluid being pumped, vent means connected to said motor casing to vent gas in said cavity, said vent means including flame-arresting means to prevent the passage of flame through the vent means and valve means operable in response to a predetermined pressure increase in said casing to close said vent means and thereby prevent the passage of fluid through said vent means. 